1. Field of the Invention
The present invention relates to an optical data recording/reproducing apparatus for recording/reproducing data in/from an optical data recording medium such as an optical disk and, more particularly, to an optical head used therefor.
2. Description of the Related Art
A technique of reproducing data recorded on an optical disk by irradiating a light beam on the optical disk and detecting light reflected thereby is widely used for a CD (Compact Disk) apparatus, an LD (Laser Disk) apparatus, and the like. In such an optical disk reproducing apparatus, a light beam emitted from a light source such as a semiconductor laser is focused/irradiated on the recording surface of an optical disk via an objective lens, and light reflected by the optical disk is detected by a photodetector, thereby reproducing data recorded on the optical disk.
As attempts have been made to increase the recording density of optical disks, optical disks have been developed on the basis of standards different from those of conventional optical disks. For example, the size of a pit as a unit in recording data is about one micron at present, but it is highly possible that the size of a pit is reduced to a value on the submicron order.
The recording density of an optical disk is determined by the size of a beam spot for a recording/reproducing operation, which is irradiated on the optical disk by a pickup (optical head) to read a very small pit formed in the optical disk to record data.
The size of this spot is determined by the wavelength of a light beam emitted from a laser used and the NA (Numerical Aperture) of an objective lens, and is given by
spot size=k.times.laser wavelength/NA PA1 where k is a constant.
When, therefore, data is to be read from an optical disk having a higher recording density by using a small spot, a laser having a short wavelength or a lens having a large NA must be used.
A conventional data recording/reproducing apparatus is designed to have only one objective lens. For this reason, data cannot be read from a high-density optical disk by using a pickup having an objective lens corresponding to a conventional optical disk.
More specifically, assume that a pickup for a high recording density uses an objective lens having a large NA. In this case, if an optical disk tilts with respect to the objective lens, the disturbance of a spot is large. For this reason, the pickup cannot be commonly used for conventional and new optical disks in many cases. That is, the allowable values for, e.g., the warp of an optical disk are large according to the conventional standards, but those for a new optical disk are small. Consequently, data cannot be read from an optical disk having large warp.
Note that the disturbance of a spot is influenced by the thickness of an optical disk. With a decrease in the thickness of an optical disk, the disturbance of a spot is reduced when the optical disk tilts. For this reason, a thin optical disk is used as a substrate for a high-density optical disk in some case.
With respect to the same optical disk, different specifications may be set for optimal objective lenses in a recording operation and a reproducing operation, respectively. Since a conventional apparatus has only one objective lens, the apparatus cannot cope with such a case.
In addition, according to new standards, there are disks having substrates which are different in thickness from each other. For this reason, it is possible that a new apparatus cannot record or reproduce data on or from a disk based on the conventional standards.
Consider the thickness of a disk substrate as a standard. An optical disk of this type is generally designed such that a reflecting film is formed on a transparent substrate (to be referred to as a disk substrate hereinafter) on which data is recorded in the form of pits, and a protective layer is formed on the reflecting film. A light beam is irradiated from the disk substrate side onto the reflecting film as a recording surface. In this case, the reproduction characteristics change depending on the thickness of a disk substrate. FIGS. 1A and 1B show changes in transmission wavefront aberration with tilting of optical disks with respect to an objective lens. FIG. 1A shows a case wherein the thickness of a disk substrate is 1.2 mm. FIG. 1B shows a case wherein the thickness of a disk substrate is 0.6 mm. As is apparent from FIGS. 1A and 1B, even with objective lenses having the same NA, the transmission wavefront aberration caused by a disk tilt is smaller in the thinner disk substrate, and a focused spot on the recording surface exhibits good focusing characteristics. Consequently, a reproduction signal having good quality can be obtained with the thinner disk substrate. For this reason, an optical disk apparatus using an optical disk having a thin disk substrate has been developed. As optical disks constituted by disk substrates having different thicknesses are developed, there naturally arise demands for reproducing data from these optical disks by using the same apparatus.
In some apparatuses proposed (e.g., Japanese Patent Disclosure (KOKAI) Nos. 4-372734, 5-266492, and 62-66433), a parallel flat plate is inserted between an optical disk and an objective lens to properly reproduce data recorded on a plurality of types of optical disks constituted by disk substrates having different thicknesses. In such an apparatus, one of parallel flat plates having different thicknesses is inserted in the optical path between an optical disk and the objective lens depending on the thickness of the optical disk subjected to reproduction processing in such a manner that the sum of the optical thickness of the parallel flat plate and that of the optical disk becomes equal to the design lens load of the objective lens. With this operation, data can be stably reproduced from the optical disk while the transmission wavefront aberration is always kept small.
In an apparatus using such a parallel flat plate, it is important that a parallel flat plate is inserted in the optical path without tilting the plate with respect to the optical disk and the objective lens. That is, a high precision is required for a moving mechanism for each parallel flat plate. In addition, if a parallel flat plate tilts with respect to the objective lens, a coma is caused. Even if, therefore, the spherical aberration is reduced by the insertion of the parallel flat plate, the shape of a focused spot is not improved. Furthermore, since the space (working distance) between the objective lens and an optical disk is small, it is very difficult to insert a parallel flat plate in this space.
In another apparatus proposed (Japanese Patent Disclosure (KOKAI) No. 5-241095), a parallel flat plate is inserted between the objective lens and the light source for the same purpose as described above. In some other apparatuses proposed (e.g., Japanese Patent Disclosure (KOKAI) Nos. 5-54406, 5-205282, and 5-266511), a compensating lens is inserted between the objective lens and the light source to properly reproduce data from various types of optical disks constituted by disk substrates having different thicknesses. According to these proposals, when the thickness of an optical disk contradicts the design lens load of the objective lens, a spherical aberration is caused, and a beam spot focused on the recording surface of the optical disk increases in size. As a result, data cannot be accurately reproduced. For this reason, the wavefronts of a light beam incident on the objective lens are adjusted to prevent a spherical aberration so as to form a small beam spot, thereby realizing stable reproduction of data from the optical disk.
As a means for preventing a spherical aberration, a wavefront correcting lens like a concave lens is used in Japanese Patent Disclosure (KOKAI) No. 5-54406; a compensating lens made of, e.g., a liquid crystal material, in Japanese Patent Disclosure (KOKAI) No. 5-205282; and a correcting lens constituted by a plurality of lens elements having variable gaps, in Japanese Patent Disclosure (KOKAI) No. 5-266511.
In an optical disk apparatus, however, the objective lens moves in the direction of the optical axis while following the warp of an optical disk. Therefore, in the method of adjusting the wavefronts of a light beam incident on the objective lens to cancel out the spherical aberration, if the warp of the optical disk is large, a change in the curvature of a light beam incident on the objective lens cannot be neglected. As a result, the spherical aberration cannot be satisfactorily canceled out.
In still another apparatus proposed (Japanese Patent Disclosure (KOKAI) No. 4-95224 corresponding to U.S. Pat. No. 5,235,581), a plurality of optical heads respectively corresponding to optical disks constituted by disk substrates having different thicknesses are selectively used to properly reproduce data recorded on each of these optical disks. More specifically, each optical head includes a semiconductor laser as a light source, an objective lens, and a photodetector. In addition, in order to selectively use these optical heads, head moving mechanisms are arranged in correspondence with the respective optical heads to move a selected optical head along the radial direction of an optical disk.
However, with these optical heads and head moving mechanisms arranged in correspondence with the thicknesses of disk substrates, the overall arrangement of the optical disk apparatus is very complicated and large in size, thus impairing the essential merit that data can be read from optical disks constituted by substrates having different thicknesses, i.e., based on different specifications, by using one optical disk apparatus.
On the other hand, as a means for stably reproducing data from a plurality of types of optical disks having different recording densities, an apparatus designed to variably change the focal length of the objective lens has been proposed (Japanese Patent Disclosure (KOKAI) No. 5-54414). This apparatus uses a liquid crystal lens designed to variably change the focal length by electrically controlling the curvature of the lens in which a liquid crystal is sealed. When the density of the data recorded on an optical disk is high, the focal length of the lens is shortened. With this operation, stable reproduction of data from optical disks having different recording densities is always performed. Since the aperture of the lens does not change, the NA increases with a decrease in focal length, and a small beam spot can be formed on the recording surface of an optical disk.
This method may be practical under the condition that the NA of an objective lens is relatively small. However, since an objective lens having a large NA (e.g., NA=0.45 for a CD; NA=0.55 for an LD) is used to focus a small beam spot on an optical disk, it is very difficult to change the surface shape of the liquid crystal lens into a shape having a small transmission wavefront aberration.
As described above, since the conventional optical head has only one objective lens, the optical recording/reproducing apparatus cannot properly cope with a case wherein a plurality of data recording media based on different specifications associated with, e.g., recording density, allowable warp amount, and substrate thickness, are to be used, or a case wherein different specifications are set for optimal lenses in a recording operation and a reproducing operation, respectively, with respect to the same data recording medium.
Note that a plurality of special pickups (optical heads) using special objective lenses complying with the respective standards and specifications may be prepared to be selectively used. In this case, however, the apparatus undergoes an increase in cost as well as an increase in size. Therefore, this arrangement cannot be practical.
All the conventional methods known as techniques of properly reproducing data recorded on a plurality of type of optical disks constituted by disk substrates having different thicknesses by using one optical disk apparatus pose practical problems.
More specifically, in the method of inserting a parallel flat plate between an optical disk and an objective lens, since the parallel flat plate is inserted in an optical path so as not to tilt with respect to the optical disk and the objective lens, a high precision is required for a moving mechanism for the parallel flat plate. If the parallel flat plate tilts with respect to the optical disk and the objective lens, a coma is caused to degrade the shape of a focused spot. In addition, it is very difficult to insert the parallel flat plate in the space between the objective lens and the optical disk.
In the method of inserting a parallel flat plate or a compensating lens between an objective lens and a light source to adjust the wavefronts of a light beam incident on the objective lens so as to prevent a spherical aberration, since the objective lens moves in the direction of the optical axis while following the warp of an optical disk, a change in the curvature of a light beam incident on the objective lens cannot be neglected if the warp of the optical disk is large. For this reason, the spherical aberration cannot be satisfactorily canceled out.
Consider the apparatus in which a plurality of optical heads, each having a semiconductor laser, an objective lens, and a photodetector, are arranged in correspondence with optical disks constituted by substrates having different thicknesses, and each optical head is moved along the radial direction of an optical disk by a corresponding special head moving mechanism to be selectively used. Since this apparatus requires a plurality of optical heads and head moving mechanisms equal in number thereto, the overall arrangement of the apparatus is complicated and large.
In the apparatus using a variable focus lens, e.g., a liquid crystal lens designed to variably change the focal length by electrical control, to stably reproduce data from a plurality of types of optical disks having different recording densities, when an objective lens having a large NA is used to form a small beam spot, it is very difficult to change the surface shape of the liquid crystal lens into a shape having a small spherical wavefront aberration.